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The Lower Cretaceous Muddy formation in northeastern Converse and northwestern Niobrara Counties, Wyoming, can be subdivided into informal lower, middle, and upper mappable units. The lower unit, predominantly sandstone and siltstone, is interpreted to be a subtidal channel-fill sequence. The channels were incised into the underlying Skull Creek Shale. The middle unit consists of mudstones and sandstones deposited on a broad intertidal flat. The upper unit is composed almost entirely of marine mudstones. The occasional sandstones that are found in the upper unit are thought to have been deposited as offshore bars in an inner to middle-shelf position.
The lower and middle units of the Muddy developed during a marine regression that began at the end of Skull Creek deposition. The upper unit reflects transgressive conditions that continued into the time of Mowry deposition.
Oil has been produced from the Muddy formation at several fields in the study area; Steinle Ranch field is the most significant.
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